Targeting the DIO3 enzyme using first-in-class inhibitors: a new paradigm in ovarian cancer treatment

Introduction: The enzyme deiodinase 3 (DIO3) contributes to cancer proliferation by inactivating the tumor-suppressive actions of thyroid hormone (T3). We established DIO3 involvement in the progression of high-grade serous ovarian cancer (HGSOC), suggesting this enzyme as a potential target for inhibition.

Methods: Compounds were designed using medicinal chemistry. DIO3-mimic assay, examining T3 iodine removal, was used as an alternative for physiological DIO3 enzyme activity. Cell number and apoptosis were assessed in an array of human HGSOC cell lines (FACS). Endogenous T3 levels were determined by ELISA. Toxicity was evaluated in mice/rats and efficacy was determined in ovarian cancer xenograft mice.

Results: DIO3 contains a selenium catalytic core which removes the iodine atoms in T3, we therefore designed a series of compounds containing a iodine atoms as scaffold, which may compete with endogenous T3 on the enzyme catalytic core. As active DIO3 protein is not commercially available, we assessed the inhibition capabilities of these novel compounds using a DIO3 mimic, as a proxy for evaluating specific targeting the endogenous enzyme. This mimic contains two selenol moieties and behaves similarly to the physiological DIO3 enzyme. Efficacy in human HGSOC cell lines identified molecules which demonstrated attenuated cell counts and induction in apoptosis. Compounds which were inactive or produced toxic results in DIO3-negative cells were excluded. Two compounds demonstrated specificity towards endogenous DIO3, as assessed by measuring endogenous T3 levels in ovarian cancer cells incubated with the drugs. Female xenograft mice were treated with both drugs for 16 days by IP administration (50mg/kg). Vehicle injected animals were used as control. Tumor volumes at the final day of the study (day 23) indicated ITYR-DBRMD as the lead compound. This molecule demonstrated a high safety profile in female mice. Analysis of signaling in both cell lines and tumors treated with ITYR-DBRMD suggested that the anti-tumor effects are mediated by downregulation in pro-cancerous proteins, the majority of which are known to be repressed by T3.

Discussion: By designing a novel small molecule that specifically targets the DIO3 enzyme we present a new treatment paradigm for ovarian cancer and potentially other DIO3-dependent malignancies